In the papermaking process, a papermaking machine is used for making a fiber web, such as a paper web, from a fiber suspension. The fiber suspension is typically in the form of fibers that are suspended in water. The fiber suspension is introduced into a headbox, at the wet end of the machine. Headbox apparatuses of such type are disclosed in, for example, from U.S. Pat. No. 4,087,321.
The quality of paper and the board forming depends significantly upon the uniformity of the rectangular jet generated by the headbox. High quality typically means good formation, uniform basis weight profiles, uniform sheet structure and high sheet strength properties. These parameters are affected to various degrees by paper fiber distributions, fiber orientations, fiber density and the distributions of fines and fillers. Thus, separation or fractionation of fibers into two or more fractions that are relatively enriched in longer or shorter fibers is an important step of the papermaking process, because it allows for the efficient use of fiber properties. Fiber fractionation allows an optimized use of raw materials, increases production versatility, and contributes to waste and energy consumption reduction.
Various technologies have been devised during the past forty years to fractionate wood pulp fibers. Pressure screen systems, which fractionate fibers based on fiber length, are generally perceived as the most successful technology on a commercial standpoint. It is also known to place a vertical partition within a headbox for the purpose of deflocculating the fiber suspension. For example, a stratifying headbox or multi-layer headbox having a single headbox converging nozzle with a separate cross machine distribution channel for each layer is disclosed in U.S. Pat. No. 4,141,788 Each suspension package (layers) is separated throughout the headbox nozzle by means of sheets or plates. In multiple headbox forming, a number of headboxes are arranged so as to form a sheet that contains multiple layers.
Attempts to establish uniform paper stock flow in the headbox component, particularly the nozzle chamber, and to improve paper fiber orientation at the slice output of the headbox also include using a diffuser installed between the headbox distributor (inlet) and the headbox nozzle chamber (outlet). The diffuser block enhances the supply of a uniform flow of paper stock across the width of the headbox in the machine direction (MD). Such a diffuser box typically includes multiple conduits or tubular elements between the distributor and the nozzle chamber which may include step widening or abrupt opening changes to create turbulent flows for deflocculation or disintegration of the paper fiber stock to ensure better consistency of the stock. See for example, U.S. Pat. Nos. 5,792,321, 5,876,564, 6,153,057, 6,303,004, 6,406,595, 6,368,460, 6,425,984, 6,475,344, and published application no. US2002/0117285.
Further, it is known to place a mechanical device within the headbox for the purpose of agitating the fiber suspension and thereby deflocculating the fiber suspension. For example, a method for generating fine scale turbulence of the fibers within the stock as it passes through the headbox is disclosed in U.S. Pat. No. 3,853,694. The method consists of welding or soldering plate(s) on the inside wall of a flow channel or headbox, wherein the plate(s) is of such material and thickness that will vibrate due to the flow of stock past the plate, with the vibration being of a higher acoustic and super-acoustic range. Such vibration aids in the dispersion of fiber networks as it passes through the headbox. See also U.S. Pat. No. 6,136,152, which discloses a headbox that includes a turbulence insert.
The feasibility of using acoustic fractionation as a means of separating fibers is disclosed in U.S. Pat. Nos. 5,803,270 and 5,979,664. These patents disclose the use of a plane ultrasonic wave field to induce lateral deflections of moving fiber suspensions in a channel flow and thereby separate fibers into two separated streams.
However, none of the above patents teach or suggest a method that comprises a method for producing stratified paper by placing at least one ultrasound transducer in the headbox, so that sound waves pass transversely through the pulp discharge and thereby separate fibers in one stratified fiber suspension stream.